The present invention relates generally to cables made by twisting one or more wires to form a strand.
It relates more particularly to the situation in which the wires are insulated electrical conductors of an electrical cable, and even more particularly, although not necessarily exclusively, the situation in which only two wires are used and the cable constitutes what is usually referred to as a twisted pair.
It is known that the twist that is conjointly applied to two wires to assemble them into a strand normally also twists each of the two wires individually on itself.
It is also known that a pair of insulated conductive wires constitutes a capacitor whose impedance depends not only on the frequency of the electrical signals conveyed, and quickly becomes non-negligible when that frequency is relatively high, but also on the capacitance between the two wires.
Finally, that capacitance is known to itself depend on the nature and the thickness of the dielectric constituting the insulative sheath of the two wires.
Because of inevitable fabrication tolerances, this thickness is not strictly constant all around the wires.
There is also inevitably some eccentricity of the conductive core of the wires relative to their insulative sheath.
If the wires are assembled with concomitant twisting of each of them, as previously indicated, the wires are in contact with each other along a generatrix which remains the same throughout their length.
The effects of eccentricity of the conductive core relative to the insulative sheath are themselves operative uniformly throughout the length of the wires, and as a result of this there may be large differences in capacitance from one pair to another Consequently the impedance can vary widely from one pair to another at comparable frequencies, which in practice leads to rejection of any pair whose impedance is too high or too low.
It has therefore been proposed to assemble the wires without hem being individually twisted.
For example, in a first system already envisaged for this purpose each of the wires to be assembled is paid out from a double-twist twisting machine operating as a paying out device and the wires are assembled by a double-twist twisting machine operating in the conventional way.
This first system therefore requires three rotating members, namely tho three double-twist twisting machines employed.
Also, production rate, or productivity, expressed as a number of twists per unit time, is equal to twice the rotational speed of the system in this case.
In another system known in the art, each of the wires to be assembled is paid out from a single-twist twisting machine and, as before, the wires are assembled by a double-twist twisting machine.
However, and just as before, this requires three rotating members, namely the two single-twist twisting machines and the double-twist twisting machine, and the production rate of the system is limited to twice its rotational speed.
A general object of the present invention is a system enabling the production of cables of satisfactory quality, in particular with regard to a relatively consistent impedance from one cable to another, but which advantageously requires fewer rotary members than and has a higher productivity than prior art systems.
It is based on the known fact that to obtain a cable of sufficient quality it is in practice possible to tolerate partial twisting, or conversely, partial backtwisting, of the wires The backtwisting of the wires can be from 25% to 50%, for example, and is preferably from 30% to 40%.
It is also based on the fact that a twisting machine has already been proposed for making large-diameter cables (or, incidentally, high-performance pairs for long distances) and is usually referred to as a lyre type horizontal pairing machine, although the path of the wires is not necessarily horizontal. It is adapted to combine two wires into a cable without any individual twisting of the wires, the twisting machine in practice using, in addition to a first paying out spool, a lyre-type assembly device within which there is a second paying out spool.
To be more precise, the present invention consists firstly of a cable-making method for making an at least partly backtwisted cable using on a common production line a twist-free assembly station at which the various necessary wires are assembled without individually twisting them followed by a twisting station at which the cable as a whole formed by the wires is twisted at least once; it also consists of any cable-making installation using a method of the above kind.
For example, in the case of fabricating a single pair, the cable-making installation of the invention very simply uses a lyre-type horizontal pairing machine for the twist-free assembly station and a double-twist twisting machine for the twisting station.
Thus only two rotary members are used, namely the lyre-type horizontal pairing machine of the twist-free assembly station and the double-twist twisting machine of the twisting station If the two rotary members turn at the same speed, the production rate of the system is three times their rotational speed.
Conjointly, assembled without twist at the exit from the lyre-type horizontal pairing machine, the two wires are then subject to only partial twisting in the double-twist twisting machine downstream of the latter.
In other words, it is just as if, overall, they were subject to some degree of backtwisting.
In practice, this depends on the relative rotational speeds of the two rotary members employed.
The rate of backtwisting can advantageously and easily be varied from one production run to another, as required.
The system preferably includes between the twist-free assembly station and the twisting station a tension measuring device controlling a braking device controlling the first paying out spool and he tension in the cable at tho exit from the twist-free assembly station is preferably substantially equal to twice the tension of the wire at the exit from the second paying out spool.
Equal tension in the two wires guarantees the geometrical quality of the resulting cable and can advantageously be obtained relatively easily and economically without knowing the tension in the wire paid out from the first paying out spool and even though, in the case of a wire that has been routed over a rotary member, a direct knowledge of that tension would require the use otherwise more complex and costly means.